This patent application relates to encapsulated active materials, and preferably relates to encapsulated catalysts, accelerators and curing agents.
In many instances, formulations which are useful as an adhesive, sealant, coating or in composite applications, such as those based on a polysiloxane, epoxy resin, polyurethane, vinyl ester resin, polyester resin, allylic resin, polysulfide resin, phenolic resin, amino resin and the like require contact with a catalytic species, accelerators or curing agent in order to achieve final cure. This cure can begin at, or slightly above, room temperature upon immediate contact between the catalytic species, accelerator or curing agent and the curable composition. Therefore, it is necessary to keep the catalytic species, accelerator or curing agent and the curable composition from coming into contact with one another until cure is desired. One commonly used approach is to formulate two part compositions in which the catalytic species, accelerator or curing agent is in one part and the curable composition is in another part. Two part compositions require shipment of two separate portions, and may require additional capital for separate containment of the catalyst, accelerator or curing agent and the curable material along with mixing equipment to mix the materials. Separate shipment and complicated equipment, such as metering and dispensing equipment, significantly adds to the cost of such a system.
It is therefore desirable to develop one-part curable compositions, which do not require shipment in two parts or complicated equipment for mixing and application. Hoffman et al. U.S. Pat. No. 5,601,761, incorporated herein by reference, discloses a method of encapsulating an active material in a coating material immiscible therewith and having a melting point or transition point above ambient temperature. The method comprises, dispersing the active material in the coating material at a temperature sufficient to melt the coating material; forming droplets of active material interspersed with the coating material; cooling the droplets to solidify the coating material to form particles; and contacting the particles with a solvent that dissolves the active material but does not dissolve the coating material, so as to remove active material from the surface of the particles.
The need to contact the particles with solvent results from the fact that a significant amount of active material is contained on the surface of the particles formed or is extractable from the particles. This significant amount of active material on the surface or which is extractable results in a lack of stability in one-part curable formulations. As a result, the patentee removed the active material at the surface by contacting the particles containing active material with a solvent for the active material. This results in stable encapsulated active material and stable compositions containing the encapsulated active material. The problem is that washing the particles after formation results in a waste of active species which is carried away in the solvent and enhanced costs due to the extra processing step of washing the particles.
Stewart et al., U.S. Pat. No. 5,120,349, assigned to Landec Polymers, incorporated herein by reference, discloses a process for encapsulating an active species, such as a herbicide, insecticide, fungicide, or fertilizer, in side chain crystallizable acrylate based polymers. These encapsulated active agents are prepared by dissolving the polymer, or precursors of the polymer in a solvent, and dispersing the mixture in a dispersant. Thereafter, in the embodiment where the precursors to the polymer are used, the necessary ingredients are added to allow the polymer to form, either before or after dispersion of the mixture. If necessary, the mixture is heated. The encapsulated active agent particles form in the dispersion and can be separated by mechanical means. Bitler, et al WO 96/27641, incorporated herein by reference, discloses the preparation of modifying agents wherein the modifying agents comprise an active chemical moiety, such as a catalyst or a curing agent, and a crystalline polymeric moiety wherein the active chemical moiety is chemically bound to the crystalline polymeric moiety. These are prepared as disclosed in Stewart. It is disclosed that these particles can be added to modify curable systems. They modify the curable systems when exposed to heat sufficient to melt the crystalline polymeric moiety and thereby bring the active chemical moiety into contact with the curable system. This system exhibits good stability but the reactivity of this system is too slow for some applications. Bitler et al, WO 98/11166, incorporated herein by reference, discloses modifying agents for curable systems, which comprise crystalline polymers containing an active chemical ingredient, which is physically bound but not chemically bound to the polymeric ingredient. The active chemical moiety and the system are similar to those disclosed in WO 96/27641. These encapsulated active species are prepared by dissolving or dispersing the active species in a hot side chain crystallizable acrylate polymer, cooling the mixture and crystallizing the mixture. The particles formed are then ground. Landec markets a product under the name Intelimer.RTM. 5012 which is dibutyltin dilaurate encapsulated by a side chain crystallizable acrylate. The active species are located at the surface of the particle and/or are extractable from the particles. In some applications the presence of the active species at the surface of the particle or the extractability of the active material from the particles results in instability of some of the formulations containing the encapsulated active species. In curable formulations this instability is exhibited by premature curing of the curable composition. This is indicated by a growth in viscosity of the composition.
What is needed is an encapsulated agent which does not require extra processing steps after formation, such as washing and is stable in a one-part formulation for extended periods of time wherein the active species can be released upon demand by application of some external phenomena such as pressure, shear or heat. In other words, the system is stable at ambient temperatures, i.e., does not undergo significant viscosity growth indicative of curing, and which cures rapidly once the system is exposed to conditions so as to release the encapsulated active agent, such as the melting temperature of the encapsulating agent.